There is evidence that the oxygen effect (oxygen sensitizes cells to ionizing radiation) is genetically determined and, therefore, there must be mutants defective in the effect. It is proposed to isolate and study such mutants in order to understand at the molecular level the mechanism(s) of the oxygen effect. Evidence has been found that indicates the effect is a post-irradiation repair process, at least for direct radiation effects. It has been demonstrated that gamma-radiation in the absence of oxygen, but not in its presence, forms an adduct between DNA, glutathione (GSH), and other cellular substances, and it is proposed that excision of these adducts post-irradiation is the mechanism of higher cell survival in the absence of oxygen. It is proposed: (1) to study the in vivo labeling of DNA with GSH and other substances in toluenized or EDTA-Tris treated E. coli and urea-treated phage T4 particles (II particle) whose DNA is labile to DNAse; (2) to use T1 DNA which has adducts produced by irradiation to isolate E. coli mutants defective in post-irradiation repair. Nonmutants will commit suicide by repairing the T1 DNA and defective mutants will be enriched among the survivors and recognized by their inability to be protected from radiation by oxygen deprivation. Ultimately, two dimensional electrophoretic gels of the cellular proteins of the mutants will be compared with the proteins of nonmutants to identify the protein(s) involved which subsequently can be studied in vitro for its action on DNA adducts. The oxygen effect is important medically because most tumor cells are at reduced oxygen tension and therefore radiation resistant. Understanding the effect at the molecular level can lead to the development of methods to lessen radiation resistance. This study involves the methodologies of genetics, biochemistry, biophysics, and molecular biology.